Induction of tyrosine hydroxylase and neuropeptide Y by carbachol: modulation with age

With aging, circulating catecholamines are elevated in both humans and animals. This may be related to the increased basal levels of dopamine beta-hydroxylase (DbetaH) and tyrosine hydroxylase (TH) mRNA levels and TH enzyme activity in the adrenal medulla of senescent compared with younger animals. Cold exposure induces TH and DbetaH mRNA, and the cholinergic pathway is believed to be involved in the cold-stimulated increase in TH expression in the adrenal medulla. However, TH gene expression in the senescent rat is resistant to stimulation by cold exposure, suggesting that the cholinergic pathway may be impaired with age in the adrenal medulla. To investigate this possibility, we administered carbachol (0.5 mg/kg i.p., every 12 hours for 3 consecutive days), a mixed nicotinic-muscarinic agonist, to young (4-month-old) and senescent (24-month-old) male F-344 rats. We examined the induction of TH mRNA, TH immunoreactivity, and TH enzyme activity in the adrenal medulla in young and old rats. In addition DbetaH and NPY mRNA levels were determined in the adrenal medulla with or without carbachol administration. Basal levels of TH mRNA, TH immunoreactivity, and TH activity as well as DbetaH and neuropeptide Y (NPY) mRNA were 1.5- to 4-fold greater in the adrenal medullae of old rats compared with young rats. Carbachol administration increased TH mRNA, TH immunoreactivity, and TH activity as well as DbetaH and NPY mRNA to the same or a greater extent in the senescent compared with the young rats. The present study indicates that the cholinergic induction of TH or DbetaH are not impaired with age, and that senescent rats retain the capacity to respond to carbachol stimulation. The present findings cannot explain why the adrenal medullae from senescent rats are resistant to the cold-induced elevation of TH mRNA and TH activity observed in young rats.

Exercise training reverses the age-related decline in tyrosine hydroxylase expression in rat hypothalamus

Tyrosine hydroxylase (TH) is the rate-limiting enzymatic step in the catecholamine biosynthesis pathway. Some studies have demonstrated that aging is associated with a decrease in TH activity and TH mRNA in rat hypothalamus. We previously demonstrated that exercise training can decrease TH gene expression in the adrenal medulla of young but not senescent rats. This study was designed to examine the effects of endurance training on the TH expression in hypothalamus with aging. To this end, we assessed TH mRNA, TH immunoreactivity, and TH activity with or without exercise training. Young and old F-344 female rats were trained by treadmill running for 8 weeks. All parameters examined were significantly lower in hypothalamus of old (25-month) compared with young (5-month) control animals (p < .05). Exercise training significantly elevated TH mRNA (n = 5-7 in each group), TH immunoreactivity (n = 5-8 in each group), and TH activity (n = 12-13 young groups and n = 6 old groups) in the hypothalamus of old animals (p < .05), but there was no significant change in any of these parameters in young animals following training. These data indicate that endurance training can reverse the age-related decline in catecholamine biosynthesis in the hypothalamus.

Induction of tyrosine hydroxylase by forskolin: modulation with age

With aging, circulating catecholamines are elevated in both humans and animals. This may be related to the increased basal levels of tyrosine hydroxylase messenger RNA (mRNA) levels and tyrosine hydroxylase enzyme activity in the adrenal medulla of senescent compared with younger animals. In addition, tyrosine hydroxylase gene expression in the senescent rat is resistant to further stimulation by cold exposure as compared with younger animals. Collectively, these observations suggest either that tyrosine hydroxylase expression is already maximally stimulated in senescent rats or that tyrosine hydroxylase gene induction pathways are impaired with senescence. To help distinguish between these possibilities, we examined the induction of tyrosine hydroxylase mRNA, tyrosine hydroxylase immunoreactivity and tyrosine hydroxylase enzyme activity in the adrenal medulla following forskolin administration to young and old F-344 rats. Forskolin at doses of 1.8 and 3.5 mg/kg increased tyrosine hydroxylase mRNA levels 2.5-fold in adrenal medulla from young rats but did not increase either tyrosine hydroxylase immunoreactivity or tyrosine hydroxylase enzyme activity 5 h after administration. Prolonged treatment with forskolin (3 doses, 12 h apart) increased tyrosine hydroxylase mRNA levels and tyrosine hydroxylase immunoreactivity and tyrosine hydroxylase enzyme activity. In senescent rats, the baseline level of tyrosine hydroxylase mRNA was more than 2-fold higher compared with young rats. A single injection of the lower dose of forskolin increased tyrosine hydroxylase mRNA levels by the same increment in senescent as compared with young rats. These data indicate that the tyrosine hydroxylase gene in the adrenal medulla from senescent rats is still capable of further stimulation.